Tracked vehicles are known for use on land and for use in the subsea environment. Such vehicles are usually carried on endless articulated tracks which may, for example, be driven by sprocket wheels engaging internal parts of the endless track. Articulated vehicle tracks have the advantage of spreading the weight of the vehicle over a large contact area.
In the subsea environment, especially in connection with laying of pipes or cables in a trench vehicles with endless articulated tracks have some disadvantages. For example, when, during trenching operations, it is necessary to correct the course of the trenching vehicle (i.e. to change its direction) conventional skid steering reduces the tractive effort of the vehicle, and can cause sinkage of the tracks into the seabed, and loss of trench cutting performance. Conventional skid steering changes the relative speed of the articulated tracks at opposed sides of the vehicle in order to effect the change in direction.
Skid steering often causes pitching of the vehicle which can result in disadvantageously high loading of the trench forming tool carried by the vehicle and also problems in the grading of the trench (i.e. the inclination of the trench base). These problems can also occur as the tracked vehicle passes over objects (for example rocks or debris) on the seabed which may similarly cause pitching of the vehicle.
A further problem can occur when a subsea trench cutting apparatus is lowered from a service vessel (for example) to the seabed. If the approach speed of the apparatus is too great, the apparatus can be damaged on contact with the seabed.
The present invention seeks to overcome or alleviate some or all of the above problems.